1. Field of the Invention
The present invention relates to a process for molding a thermoplastic resin incorporated with a large amount of an inorganic filler.
2. Prior Art
It has been known that artificial stones can be made by pressing and heating a mixture consisting of a large amount of finely divided stones with the fluid prepolymer of a thermosetting resin, such as for example, unsaturated polyester resin, epoxy resin and melamine resin. It is also known that such such stones are made by heating and curing a similar mixture in which finely divided stones have been uniformly dispersed in the liquid prepolymer. For example, in this case, the stones are dispersed by shaking, or by pouring the fluid prepolymer into spaces between finely divided stones and then curing same.
In these conventional methods of preparing artificial stones, the curing by polymerization of the thermosetting resin takes a great deal of time and also a lot of labor is required to pour the molding material into a metal mold and take out the molded objects. Moreover, what is worse in those conventional prior art methods, when the slightest mistake is committed in conditioning, the molding material tends to have air bubbles in it or crack, graze and shrink during curing. This makes it difficult to produce standardized products.
Studies have been conducted on the method of mass producing artificial stones and other shaped articles by using thermoplastic resin, instead of thermosetting resins, by means of an extruder. However, no satisfactory results have been obtained as yet.
Since the thermoplastic resin material used for the foregoing purposes is generally in the form of powders or pellets, an intimate mixture would hardly be made sufficiently and with a large quantity of an added inorganic filler by mere heat and pressure. Moreover, any molding machine fed with such a composition often becomes unworkable. Furthermore, even if this composition works, it cannot give a product of superior quality. For example, if a mixture of polyethylene pellets with finely divided stones is put into a metal mold, pressed with heat and then cooled, a molded product of mediocre quality will be obtained; but, because of the poor intimacy of the mixture of the resin with the filler material, the texture of the product would not be good enough. If finer pieces of broken rocks are employed, the degree of intimacy of the mixture of the resin with the filler material would be somewhat improved, but there will still be uneveness in the permeation of the resin material into spaces between the pieces of rocks. Thus, products having a fine texture would not be obtained with the filler uniformly distributed. The same will be the case even when varied kinds of resin are used.
Further, when the molding composition is applied to an ordinary extruder whose cylinder and metal mold have been well heated, the machine will be overloaded and become unworkable in a short time after start of the operation. This will be due to the lack of lubricity of the filler and the fact that the resin does not melt sufficiently. In order to overcome these defects, it has been proposed to heat the mixture of resin pellets and the filler in a separate preheating vessel, and then feed them to the extruding machine. In this case, however, if the preheat temperature is no higher than the melting point of the resin itself, the extruding machine would become overloaded also in a similar manner as stated above. On the other hand, if the preheating temperature is made as high as about the melting point of the resin, the resin pellets will become fused with each other by the slightest mixing movements in the preheating vessel. Thus, it would become difficult for the material to be poured into the extruder; if poured with force, the molding material tends to adhere closely to the screw between the feeding and compression zones, leaving the machine running in idle. Only when the preheating temperature is made a little higher than the melting point of the resin, the extruder will remain workable for some time, but thereafter the amounts being extruded become gradually smaller, until at last the resin is fused to the screw, making the operation of the extruding machine entirely impossible. Thus, no satisfactory results may be given in any of these cases.